T.B.E. 208 C.L. Meter Repair

[BlownUpCapacitor]

So I recently acquired a T.B.E. 208 C.L. Meter for cheap at my local e-recycle. It doesn't work, but it powers on and displays some random digits on the display.

What works:

The range selector from what I can tell

The PSU

Range display and controller unit

component selector system

display hold

What doesn't work:

Its actual ability to measure components

Some ranges turn off the display for some reason. I think I've mostly tracked down the reason. discussed later.

zero button; pressing the button does nothing. I don't see any voltage at the contacts either so that could be an indicator for something.

Some more notes:

It has a BNC output, and when connecting it to my scope, the voltage on the BNC output varies depending on the range. The BNC is connected to 3 wires internally; one wires goes to the input of a TL500CN ADC unit which is attached to the display unit. Presumably, this LC meter produces a display depending on the voltage at this BNC. The other two wires lead to other PCBs, one to a mini PCB that appears to be in charge of the zero function because of its connection to the zero switch. The last wire leads to the main board where it is connected through some resistors to the output of an OP200 precision op-amp.

The truth table for the display to be either on or off depends on the range, and the truth table is this:

Capacitor: 10pF  100pF  1000pF 10nF   100nF  1000nF 10uF
                 on      off       off       off       off        off       off
VoltsBNC: -1.10  +3.70  +4.00  +3.70  +4.00  +3.55  +3.55

Inductor:  10uH   100uH 1000uH  10mH    100mH                        1000mH     10H
                 on       off        on         off      on(random though)         on             on
VoltsBNC: -3.32  +4.75  +0.81*  +3.95*  +0.560*                        +2.06*     -2.20*

* indicates randomly varying voltage (not mains AC)

The voltages on the inductor scale is quite chaotic and is subject to change depending on how close my hands are to the input. Does not seem to correlate to operation though.

About the varying voltages, when the voltage at the BNC output (and thus subsequently the voltage at the output of the op-amp, the zero PCB, and the ADC for the display) goes above a certain point above zero, the display turns off. This can be mitigated by attaching a 50 ohm terminator to the BNC output thus loading this voltage down so that the display is always on no matter the range. The source of this voltage appears to be from the OP200 precision op amp. Initially suspecting a faulty op-amp, I swapped it with an LM358 for testing but that produced the same results. So the op-amp appears to not be the problem. I have tried to trace the signal back but without a schematic, it has proven difficult.

provided a video to perhaps better clarify the issue.

I will update soon.

Photo files too big:
https://drive.google.com/drive/folders/1iV1_ree4nA7_8KBTLtXWVrmF-h4sBgEX?usp=sharing

Near the beginning of the video, I touch the vertical position while showing the erratic voltage changes on the BNC, so don't trust any voltage readings I give after that. Video: https://youtu.be/mAk5KPfCRik

[Swake]

I couldn't find anything about this device and nothing about T.B.E. unless this is or is related to Teledyne (and Teledyne Brown Engineering).

The screen going out might be because of an overload or out-of-limits situation.

You could try finding out if the IC's are doing something that is coherent. For example there are all sorts of op-amps on the main board, you could scope inputs and see if the output makes sense. The DG506A is a switch. I guess to select the range. is it switching as expected?

Else you'll have to reverse engineer parts of that thing to better understand what is going on.

It is going to be a challenge

[BlownUpCapacitor]

Thanks for your reply Swake.

On the topic of range switching, the functions appear to be working, at least from what I can tell based on my limited testing. Each push of the range button gives a small and quiet bding from the relays switching, however, switching to some ranges do not give me a bding. This may be normal, or the range switching board isn't ranging the relays properly, or maybe there is a relay with an open coil. More about the relays themselves, the relays might also have bad contacts, leading to bad range connections and potential bogus readings on the screen. The random varying voltage does resemble one of a bad contact as well. I remember repairing a Tek 7B85 time-base plug-in and a bad potentiometer with poor contacts produces similar results.

Upon probing the OP-AMP that connects to the BNC, the one that has been outputting the strange voltage, its inputs appear to be doing the same, and switching it out with an LM358 gave the same results. It appears that the input going into that op-amp seems to be from the range switching board, so I may reverse engineer that PCB to figure out what does what.

Speaking of reverse engineering, I reverse engineered the simplest PCB in the unit, the "zero" PCB; I have attached a PNG file of my reverse engineered schematic. From what I can see, it just amplifies and inverts the input signal? But no negative voltage at the "output".

It seems like I have a lot more reverse engineering ahead of me. The meter seems to be good quality and well designed. My suspicion is that someone tried to measure the capacitance of a charged capacitor and fried the front-end and maybe some other stuff further back.

[BlownUpCapacitor]

Doing some more simple testing, the display seems to turn off only when the thing "works". I define this "work" by the sine wave oscillations that appear at the terminals when a component is inserted, in this case an inductor. When the unit "works", the voltage at the output also goes high.

The varying voltages seem to have been normal, just means it was "working". I however cannot figure out why the display turns off when the output goes high. Is this a problem with the display unit?

I'm going to try and reverse engineer the display unit now.